High mechanical and heat performance optical cable

ABSTRACT

A high mechanical and heat performance optical cable for aerial or underground applications comprises a dielectric central member, a plurality of polymeric tubes helically arranged around the dielectric central member, a plurality of optical fiber loosely housed in the polymeric tubes and separated from the polymeric tubes by a filling, a binder comprising polymeric tapes helically arranged around the tubes, a peripheral pulling element arranged around the polymeric tubes, a flame resistant external sheath surrounding the peripheral pulling element, and metallic elements helically arranged around the flame resistant external sheath to form an external ring.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention refers to an optical cable, preferably for aerialapplications. However it can also be used underground and has highmechanical and heat performance properties because of its construction.

Optical cables in aerial networks are installed in electrical powerdistribution networks of medium and low tension or in its own pole line,in urban and rural areas. In rural areas, accidents frequently occurwith optical cables, due to fire caused by natural or forced burning oflocal vegetation and also caused by hunter's shots.

Other important aspects of aerial optical cables are the installationconditions, i.e., maximum distance between poles and sag, climaticconditions, etc., which must be seriously taken into account.

Therefore, an object in the design of aerial optical cables is to meetthe above mentioned installation conditions, as well as, when necessary,to withstand heat conditions caused by the above-mentioned fires or themechanical damage caused by hunter's guns, without damaging the fibersor altering their optical characteristics.

In the market there are different types of totally dielectric aerialoptical cables, which have several advantages over the traditionalsolutions of metallically sheathed cables, which use corrugated orsmooth aluminium or steel tapes. However, principally from theresistance point of view against fires and gun shots, the dielectricsolutions exhibit a low performance and complexity in the productionprocess.

Furthermore, the construction of totally dielectric aerial opticalcables, to be installed with large spans, requires a great number oftextile pull elements so as to resist the mechanical stress in theclimatic and more critical installation conditions (e.g. maximum windforce, minimum temperature, etc.). This excessive number of textileelements, in addition to presenting difficulties in the cable'smanufacturing cycle, increases the unit weight as well as the cable'soutside diameter, making its dimensions more critical.

An alternative, having high efficiency and proven through variouspractical applications, is the use of lightning conductors opticalcables (OPGW). These OPGW cables have exceptional optical and mechanicalcharacteristics, since they have around their dielectric nucleus, wherethe optical fibers are housed, an aluminium tube and surrounding it oneor more layers of metallic wires.

These cables are preferably indicated for new aerial installations inpower transmission lines or replacing conventional lightning conductorcable, offering one of the highest security coefficients againstaccidents caused by different factors, including fire and gun shot.

These cables also have been used in distribution networks or their ownpole lines, to take advantages of their exceptional optical andmechanical features. However, for these installations, this alternativeis not always the best solution from the economical point of view.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the invention, a highmechanical and heat performance optical cable for aerial or undergroundapplications comprises optical fibers housed with a dimensionalclearance inside protective polymeric tubes, a filling, a peripheralpulling element, an external flame resistant sheath and helicallyapplied metallic elements forming an external ring, the protection tubesbeing assembled around a central element of dielectric material, on ahelicoid structure, via fastenings of polymeric wires and binding withhelicoid shaped polymeric tapes.

The optical cable also preferably provides the use of watersuperabsorbent elements in the interstitial areas forming a dry nucleus.

Moreover, the optical cable preferably provides the use of heatresistant material in the binding such as polyaramide or fiber glasstapes.

As a result, the present invention provides an alternative, which istechnically and economically viable, of an aerial optical cable project,so as to assure that the above-mentioned optical cable works accordingto the project's conditions and, therefore, guarantee the mechanical andoptical integrity of the optical fibers, against anticipated mechanicalstress and temperature limits. It also shows better performanceregarding heat requirements resulting from fires, as well as impactscaused by gun shots.

To better understand the present invention, it will now be explainedbased on the following detailed description and accompanying drawingdisclosing an embodiment of the above-mentioned optical cable.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing shows a preferred embodiment for the cable ofthe present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

There are various structures used to house and protect optical fibersforming the optical cable nucleus. One of the structures most used iscalled loose tube type structure or fibers loose within a tube.

One or more optical fibers 1 are housed inside a polymeric tube 2, whichhas a dimensional clearance in relation to the optical fiber beam 1,that will result in a cross free play of the fibers.

The dimensions of tube 2 and the relative length of fiber 1 in itsinterior, in relation to the length of tube 2, are parameters which areestimated so as to meet the requirements of the lengthening andcontraction of the optical cable.

To form the optical nucleus, these tubes are gathered around a centralelement 7, which can be manufactured of various materials and, usually,work as the strengthening member or mechanical support. When the opticalcable has metallic elements on its outside, these central elements 7must be of dielectric material, in order to avoid electrical discharges.The gathering of the tubes 2 on central elements 7 can be screw-shaped,forming an open or close propeller. This technique is called SZgathering.

This screw-shaped structure gives the fibers 1 a radial movement margin,because of the lengthening or contraction of the optical cable.

On tube structure 2 gathered around central element 7, usually polymericwire fastenings and binding 4 with polymeric tapes, also screw-shaped,are used. They assure stability of the structure and the linking oftubes 2 to central element 7.

To protect the fibers from contact with humidity, the loose tubes 2 andthe interstitial areas 3 are filled with a viscous polymeric gel.However, the present invention also provides for the use of a drynucleus, i.e., with no gel filling in the interstitial area 3 of theloose tubes 2, which are still kept filled so as to protect the opticalfibers 1.

In this case, to protect the optical nucleus, against water penetrationand spreading in these interstitial areas 3, the present invention useswater superabsorbent elements. These materials can be used on thecentral element 7 and on the gathered tubes 2, in order to block allpossible routes. One of the ways of use, which must not be consideredthe only one, is the use of a longitudinal wire applied on the centralelement 7, positioned under the gathered tubes 2, and a tape helicallyapplied on the optical nucleus.

One of the advantages of having a dry nucleus is minimizing the weightof the cable, as well as the internal pressure caused by the fillinggel, arising from increases in temperature, in the possible situationsin which this occurs.

To offer better features regarding mechanical traction, these cables canbe reinforced by the use of elements such as high pull textile wires,which are strung over the optical nucleus or laid longitudinally on thesheath.

As an external protection for this central nucleus, called the cable'soptical nucleus, additional protections are applied which can only bedielectric of polymeric materials, metallic or compounds of metallic(typically aluminium or steel) and polymeric material. Each type ofmaterial or compound is used to offer specific features in the cablesfor specific applications.

In the present invention, the optical cable nucleus is totallydielectric, being coated by one or more layers of polymeric material andthe outermost is flame resistant. Further it is possible to use heatresistant material bindings, such as polyaramide or glass fiber tapes.

To attain a better performance of the cable under fire and heatrequirements, as well as against impacts caused by guns, as mentionedabove, the present invention provides the application over this flameresistant coated nucleus of metallic elements 8 helically applied,forming an external ring.

These metallic elements can be manufactured of various materials, suchas aluminium, steel, aluminium coated steel (alclad), etc., and can havevarious shapes, such as cylindrical, rectangular, trapezoidal, etc.

When this metallic frame is applied on the coated nucleus, techniquescan be used to assure its better adhesion on the polymeric coating,amongst them, for example, the use of adhesives or heat process.

The metallic frame is preferably also responsible for the cable'smechanical support in the most critical conditions of installation andclimatic conditions, in order to optimize the design, from thecost/benefit point of view.

The use of this optical cable in aerial networks does not have any kindof special requirement during the installation and operational phases.Therefore, it is an efficient alternative for the available solutions.

The present invention also can be applied in underground installation,inside tubing or directly buried, having as an advantage an effectiveprotection against attack by rodents, a typical problem of dielectriccables.

Although the preferred embodiments of the present invention have beendescribed and illustrated, it is obvious to those skilled in the artthat several modifications can be applied without departing from theprinciples of the invention.

What is claimed is:
 1. A high mechanical and heat performance opticalcable for aerial or underground applications, comprising: a dielectriccentral member; a plurality of polymeric tubes helically arranged aroundthe dielectric central member; a plurality of optical fibers housed inthe polymeric tubes and separated from the polymeric tubes by a filling;a binder comprising polymeric tapes helically arranged around the tubes;a peripheral pulling element arranged around the polymeric tubes; aflame resistant external sheath surrounding the peripheral pullingelement; and metallic elements helically arranged around the flameresistant external sheath to form an external ring.
 2. An optical cableaccording to claim 1, wherein the filling comprises a watersuperabsorbent element.
 3. An optical cable according to claim 1,wherein the binder comprises a heat resistant material.
 4. An opticalcable according to claim 1, wherein the binder comprises a polyaramidetape.
 5. An optical cable according to claim 1, wherein the bindercomprises a fiberglass tape.